Directory UMM :Data Elmu:jurnal:S:Soil & Tillage Research:Vol55.Issue3-4.Jun2000:
Soil & Tillage Research 55 (2000) 143±165
Weathered pumice in¯uence on selected alluvial
soil properties in west Nayarit, Mexico
J.E. Gama-Castro, E. Solleiro-Rebolledo*, E. Vallejo-GoÂmez
Instituto de GeologõÂa, Universidad Nacional AutoÂnoma de MeÂxico (UNAM), Circuito de la InvestigacioÂn Cientõ®ca,
Cd. Universitaria, CP 04510, MeÂxico D.F., Mexico
Received 28 August 1998; received in revised form 22 March 1999; accepted 24 February 2000
Abstract
Parent material and pumiceous alluvial soils, located in a tropical region of Mexico, were studied to supply information on
soil suitability for agricultural production in the context of sustainable agriculture. In recent alluvial soils, an understanding of
how soil characteristics vary with parent material and topography provides a basis for determining land utilization type, land
suitability as well as land quality. The main objective of this study was to establish the relationship of soil properties to parent
material in west Nayarit, Mexico. Field studies were initiated in 1993 by a request for technical assistance from the Comision
Federal de Electricidad. The studied soils were derived from pumice that has been reworked and mixed with detrital material
from other sources. We found that such soils have unique physical, chemical and mineralogical characteristics that are rarely
found in soils derived from other parent materials. Data for two selected alluvial soil pro®les are presented. These soils were
developed on Holocene volcanogenic pumiceous alluvial river terraces and river ¯oodplains, under current udicisohyperthermic soil-climate conditions. The agronomic properties, tillage in¯uences and fertilizer requirements of these
soils have been studied extensively. To maximize their productivity and minimize deterioration, proper management must be
based on an understanding of the unique physical, chemical and mineralogical properties. Results indicate that such soils have
physical properties that provide a good environment for deep rooting and can supply the water necessary for vigorous plant
growth. In both soils, water retention at 33 and 1500 kPa, particle surface area, calculated clay, cation exchange capacity, Al
exchangeable percentage and P retention, and the occurrence of isotropic coatings on rock fragments and peds tend to increase
in the presence of the large amounts of hydrolyzed pumice that are found in the 0.02±2.0 mm fraction. Scanning electron
microscope±energy dispersive X-ray analyses demonstrate that the coatings dominantly consist of noncrystalline material,
probably allophanic-like material as suggested by the Si/Al molar ratio of 1.0. Selective dissolution analysis reveals that these
abundant noncrystalline materials consist of ferrihydrite and allophane with an atomic ratio (AloÿAlp)/Sio of approximately
1.4. Both ferrihydrite and allophane have very large speci®c surface area and absorptive capacity that make a signi®cant
contribution to the overall properties of these soils. X-ray diffraction analysis and transmission electron microscope
observation show that the major group of crystalline clay minerals in the upper section of studied soils are tubular and
spheroidal halloysite. These soils were classi®ed as Vitrandic Udi¯uvents according to Soil Taxonomy. # 2000 Elsevier
Science B.V. All rights reserved.
Keywords: Weathered pumice; Volcanogenic alluvium soils; Particle-size class; Vitrandic Udi¯uvent
*
Corresponding author. Av. Acoxpa 52, Edif. B depto 2, CP 14380, 04510 MeÂxico D.F., Mexico. Tel.: 52-55-62-4286;
fax: 52-56-22-4317.
E-mail addresses: solliro@geologia.unam.mx, solleiro@geol-sun.igeolcu.unam.mx (E. Solleiro-Rebolledo)
0167-1987/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 7 - 1 9 8 7 ( 0 0 ) 0 0 1 1 4 - 8
144
J.E. Gama-Castro et al. / Soil & Tillage Research 55 (2000) 143±165
1. Introduction
More than half of the volcanic ash soils as well as
soils with andic properties, including those containing
pumice, are located in tropical regions where climatic
conditions are signi®cantly different from those of
temperate regions (Shoji et al., 1993a). Since climate
greatly affects soil-forming processes, soils formed in
tropical regions are expected to differ considerably
from those formed in temperate regions. Although
much information is available about pumice soils
(Egawa, 1977; Maeda et al., 1978; Leamy et al.,
1980; Lowe, 1986; Wada, 1989; Kawasaki et al.,
1991; Dahlgren et al., 1993; Jongmans et al., 1994),
unfortunately it is mostly limited to soils formed in the
middle latitudes or temperate regions.
In Mexico, volcanic ash soils and pumice-derived
soils did not receive attention until the middle of this
century. This recognition stemmed from an urgency to
determine the potential agricultural productivity of
these soils because of the increased demand for food
production. However, in tropical regions of Mexico,
the properties, productivity and classi®cation of soils
formed in pumice or material derived from pumice
that has been reworked and mixed with material from
other sources (volcanogenic pumice alluvium (VPA))
are still a matter for investigation and discussion
(Gama-Castro, 1996). These soils have a great potential for agricultural production as illustrated by the fact
that many of the most productive tropical regions of
Mexico are located on river ¯oodplains close to
volcanic ®elds.
In tropical regions, pumice and other pyroclastic
materials weather rapidly, frequently resulting in the
formation of noncrystalline minerals. Noncrystalline
materials common in VPA include allophane, allophane-like materials, opaline silica and ferrihydrite.
These noncrystalline materials typically occur as coatings on the skeleton (Dahlgren et al., 1993). Because
of the large speci®c surface and high proportion of
reactive sites, a small amount of these noncrystalline
materials can make a disproportionately large contribution to the physical and chemical properties of
these soils. In fact, properties such as water retention
and cation exchange capacity are closely related to
molecular weathering ratio (SiO2/Al2O3) which is a
weathering index in volcanic ash soils (SolleiroRebolledo, 1998).
Weathered pumice-derived soils have morphological, physical, chemical and mineralogical properties
that can be attributed to the in¯uence of a combination
of factors involved in their development including: (1)
very low resistance of the ®nely comminuted pumice
to chemical weathering (Shoji et al., 1993b); (2) high
water retention and cation exchange capacities (Ping
et al., 1989; Par®tt, 1990; Henmi, 1991); (3) low bulk
density (Geist and Strickler, 1978; Bielders et al.,
1990); (4) particle-size distribution and great particle
surface area (Wada, 1986; USDA-SSS, 1994); and (5)
illuviation and neoformation of the short-range order
material, 1:1 phyllosilicates, noncrystalline aluminum
and iron oxides and hydroxides by weathering of
pumice in humid climates (Quantin et al., 1991;
Dahlgren et al., 1993).
In agreement with Osher and Buol (1998), we
consider that ef®cient and sustained use of the pumice
soils requires an understanding of how soil characteristics vary with parent material and their location. We
also assume that to maximize soil productivity and
minimize deterioration, proper management must be
practiced, based on an understanding of the unique
morphological, physical, chemical and mineralogical
properties.
The goals of the present study were: (i) to characterize the soils derived from VPA and estimate the
amount of pumice in the pro®le of selected alluvial
soils formed in the Santiago River ¯oodplain by
optical and physical means; (ii) to establish the in¯uence of weathered pumice in macro and micromorphological, physical, chemical and mineralogical
features of selected alluvial soils; and (iii) to interpret
their genesis, to classify them using Soil Taxonomy
(USDA-SSS, 1994) and to suggest revisions of Soil
Taxonomy where necessary.
2. Site conditions
The study area is situated on the ¯oodplain of the
Santiago River in the coastal plain of west Nayarit,
Mexico (Fig. 1). This area received VPA throughout
the late Pleistocene to recent period (until July 1992).
The various pumiceous deposits can be identi®ed on
the basis of sur®cial geology, stratigraphic position,
geomorphic surfaces, spatial relations through elevation and distance control and soil characteristics such
J.E. Gama-Castro et al. / Soil & Tillage Research 55 (2000) 143±165
145
Fig. 1. Location of soils derived from VPA in west Nayarit, Mexico: P33, Aguamilpa pro®le; P50, Ixcuintla pro®le.
as texture and mineralogy. The Quaternary volcano
San Juan, located on the southwest side of Tepic, State
of Nayarit, is considered the main source of the parent
pumice. Volcanic materials consist of ®ne-grained
dacitic and andesitic pumice deposits (Luhr, 1978).
Both dacitic and andesitic deposits were reworked and
intensively mixed with detrital material, poor in
weatherable minerals and nutrients (Gama-Castro
and Palacios-Mayorga, 1994) with a thickness ranging
from 70 to 150 cm. The 14C data suggest that the
oldest pumiceous deposit is from about 14 700 years
BP (Luhr, 1978).
The area presently is being weathered under a
humid, warm climate with mean January air temperatures between 22 and 238C, and mean July temperatures between 27 and 298C (Table 1). Average
annual precipitation exceeds potential evapotranspiration by about 515 mm per year (Table 1). According
to Par®tt (1990) this annual precipitation (>1200 mm)
leads to the preferential formation of allophane and
allophane-like materials when soluble Si concentrations are low. In the absence of run-off or run-on,
about 400 mm of water percolates through the soil.
The depth of weathering in the soils located in
the study area varies in response to erosion and
sedimentation rates. However, micromorphological
observations as well as mineralogical and chemical
analyses show some evidence of selective weathering
of the primary minerals within 70 cm of the soil
surface. Weathering intensity decreases with soil
depth. In many soils, the groundwater seasonally
¯uctuates, often intersecting the bottom of the C
horizon in the summer. Sometimes, reduced conditions are evident in the deeper layers, with a rH value
19 (rH is related to the redox potential Eh and
corrected for the pH according to FAO-ISRIC-ISSS,
1994).
The soils occur on ¯oodplain landforms and
some river terraces formed by the deposition of
VPA sediments. In these areas the elevations range
from 1.5 to 10 m and the slopes range from 0.5 to 3%
(Table 1).
Deforestation occurred between 50 and 70 years
ago, and the current vegetation consists of grass and
shrubs with a limited number of trees. The original
vegetation was deciduous seasonal forest (Beard,
1955).
The studied soils are among the most important
agricultural soils in the Santiago coastal plain of west
Nayarit. The production of vegetables, tobacco, beans,
rice, sugar cane, and improved pastures support the
dairy industry on these soils.
146
Soila
Horizon development
Location
Slope
(%)
Elevation
(m)
Physiography
Annual mean
temperature
(8C)
Annual mean
rainfall (mm)
Land
use
Parent material
Aguamilpa
Ap±C1±C2, a-tchb, 2Cd±3C
218420 N, 1058170 W
0.5±1.5
10
25
1267.4
Ac
Ixcuintla
Ap±C1, a-tch, 2C±3C
218460 N, 1058140 W
0.5±1.0
2
Flood plains
nearly level
Flood plains
level
26
1227.3
Ac
Volcanogenic alluvium
over detrital deposits
Volcanogenic alluvium
over detrital deposits
a
According to Gama-Castro and Palacios-Mayorga (1994).
Abrupt textural change.
c
Agricultural, tobacco, beans.
b
J.E. Gama-Castro et al. / Soil & Tillage Research 55 (2000) 143±165
Table 1
Pedon locations and site properties
J.E. Gama-Castro et al. / Soil & Tillage Research 55 (2000) 143±165
3. Methods
3.1. Soil selection and ®eld procedures
A semi-detailed survey of soils in west Nayarit,
including Yago and San Blas (Fig. 1), was conducted
over a 10-month period (Gama-Castro and PalaciosMayorga, 1994). This survey was based on mapping
soil units by air photograph interpretation combined
with a ®eld survey. The purpose of this survey was to
supply information, which assisted in taking decisions
about land use and land development planning.
During this survey, 61 soil pro®les were described
from the soil surface to the upper boundary of a speci®ed horizon or root-limiting layer. The vertical section
so de®ned is called soil control section (SCS) according to ICOMAND (1987) and USDA-SSS (1994). The
soil pro®les were described following the guidelines
of FAO-UNESCO (1994) and classi®ed according to
USDA-SSS (1994). These pro®les were sampled in
the ®eld and analyzed in the laboratory. These analyses revealed that 13 of the soils derived from VPA
had unique and complex attributes in the context of
soil production and soil management. Such agronomic
characteristics are related to absorption and adsorption
complexes, aeration, available water capacity, bulk
density, cation and anion exchange capacity, particle-size distribution as well as the presence of amorphous mineral clays in their SCS. These soils showed
high cation exchange and water retention capacities
and low contents of available elements, organic matter
and clay (Gama-Castro and Palacios-Mayorga, 1994).
Based on these previous studies, two typical pedons
representative of such soils were selected for analysis
and soil classi®cation. The pro®les characterize the
two main textural families of soils formed from
different parent materials (USDA-SSS, 1994). These
pedons are numbers P33 and P50. The two pedons are
named after localities in the study area as follows:
P33, Aguamilpa soil, and P50, Ixcuintla soil (Fig. 1).
These soils are intensively used as arable land and
represent some of the most important agricultural soils
found in the Nayarit tropical zone.
3.2. Physical analysis
All physical analyses, except water retention at 33
and 1500 kPa as well as bulk density, were made on
147
air-dried soil samples passed through a 2 mm sieve.
The methods employed are brie¯y described below.
Particle-size distribution analysis: The record of the
volume percentage of material >2 mm was estimated
in the soil descriptions. These fractions were collected
and weighed to determine the percentage of rock
fragments. The sand fractions >47 mm were collected
by sieving on a Fristch Analysette shaking apparatus
before removing the organic matter with 30% H2O2.
Fractions 0.6 or 2.0 mm) in the whole soil in studied pedons: (1) reworked pyroclastics; (2) glass
aggregates; (3) detrital sediments. US: upper section; LS: lower section.
separate strata of Holocene alluvium. Such discontinuities show the presence of intermittent alluvial
sediment deposition by Santiago River ¯ooding,
spread some hundreds or thousands of year apart.
Stratum 1, named as the upper section, contains the
earliest pyroclastic deposits of VPA and includes Ap
and pedogenetically altered C horizons. Stratums 2
and 3, the lower section, are unaltered fragmental
deposits that include the 2C and 3C layers, respectively (Fig. 2; Table 2).
In the upper section of both pedons, rock fragments
(5.0±2.0 mm in diameter) represent a signi®cant fraction of the whole soil (26±39% by weight, Table 3).
They are mainly constituted by reworked pyroclastic
sediments mixed with ordinary detrital sediments and
a certain amount of organically derived material. As
shown in Fig. 2, the detrital clastics (lithic fragments)
such as cobbles and pebbles are relatively scarce in
this section (15±17% by volume of the whole soil).
Many of these lithic fragments have lost their original
internal fabric and subrounded shape and exhibit some
compressed structures and irregular shapes, suggesting fragmentation and deformation. A great percentage of reworked pyroclastics and lithic fragments
have a red- to yellow-colored chemical alteration rind,
indicating that weathering has affected these materials.
In contrast, the layers of the lower section contain
many round and subrounded pebbles, cobbles and
some stones that constitute more than 45% by volume
of the whole soil. The fraction ®ner than 2.0 mm is
sandy as de®ned for the sandy particle-size class
(Table 3). Such layers satisfy all the requirements
for a sandy skeletal class. In addition the number of
lithics with an alteration rind is very low (
Weathered pumice in¯uence on selected alluvial
soil properties in west Nayarit, Mexico
J.E. Gama-Castro, E. Solleiro-Rebolledo*, E. Vallejo-GoÂmez
Instituto de GeologõÂa, Universidad Nacional AutoÂnoma de MeÂxico (UNAM), Circuito de la InvestigacioÂn Cientõ®ca,
Cd. Universitaria, CP 04510, MeÂxico D.F., Mexico
Received 28 August 1998; received in revised form 22 March 1999; accepted 24 February 2000
Abstract
Parent material and pumiceous alluvial soils, located in a tropical region of Mexico, were studied to supply information on
soil suitability for agricultural production in the context of sustainable agriculture. In recent alluvial soils, an understanding of
how soil characteristics vary with parent material and topography provides a basis for determining land utilization type, land
suitability as well as land quality. The main objective of this study was to establish the relationship of soil properties to parent
material in west Nayarit, Mexico. Field studies were initiated in 1993 by a request for technical assistance from the Comision
Federal de Electricidad. The studied soils were derived from pumice that has been reworked and mixed with detrital material
from other sources. We found that such soils have unique physical, chemical and mineralogical characteristics that are rarely
found in soils derived from other parent materials. Data for two selected alluvial soil pro®les are presented. These soils were
developed on Holocene volcanogenic pumiceous alluvial river terraces and river ¯oodplains, under current udicisohyperthermic soil-climate conditions. The agronomic properties, tillage in¯uences and fertilizer requirements of these
soils have been studied extensively. To maximize their productivity and minimize deterioration, proper management must be
based on an understanding of the unique physical, chemical and mineralogical properties. Results indicate that such soils have
physical properties that provide a good environment for deep rooting and can supply the water necessary for vigorous plant
growth. In both soils, water retention at 33 and 1500 kPa, particle surface area, calculated clay, cation exchange capacity, Al
exchangeable percentage and P retention, and the occurrence of isotropic coatings on rock fragments and peds tend to increase
in the presence of the large amounts of hydrolyzed pumice that are found in the 0.02±2.0 mm fraction. Scanning electron
microscope±energy dispersive X-ray analyses demonstrate that the coatings dominantly consist of noncrystalline material,
probably allophanic-like material as suggested by the Si/Al molar ratio of 1.0. Selective dissolution analysis reveals that these
abundant noncrystalline materials consist of ferrihydrite and allophane with an atomic ratio (AloÿAlp)/Sio of approximately
1.4. Both ferrihydrite and allophane have very large speci®c surface area and absorptive capacity that make a signi®cant
contribution to the overall properties of these soils. X-ray diffraction analysis and transmission electron microscope
observation show that the major group of crystalline clay minerals in the upper section of studied soils are tubular and
spheroidal halloysite. These soils were classi®ed as Vitrandic Udi¯uvents according to Soil Taxonomy. # 2000 Elsevier
Science B.V. All rights reserved.
Keywords: Weathered pumice; Volcanogenic alluvium soils; Particle-size class; Vitrandic Udi¯uvent
*
Corresponding author. Av. Acoxpa 52, Edif. B depto 2, CP 14380, 04510 MeÂxico D.F., Mexico. Tel.: 52-55-62-4286;
fax: 52-56-22-4317.
E-mail addresses: solliro@geologia.unam.mx, solleiro@geol-sun.igeolcu.unam.mx (E. Solleiro-Rebolledo)
0167-1987/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 7 - 1 9 8 7 ( 0 0 ) 0 0 1 1 4 - 8
144
J.E. Gama-Castro et al. / Soil & Tillage Research 55 (2000) 143±165
1. Introduction
More than half of the volcanic ash soils as well as
soils with andic properties, including those containing
pumice, are located in tropical regions where climatic
conditions are signi®cantly different from those of
temperate regions (Shoji et al., 1993a). Since climate
greatly affects soil-forming processes, soils formed in
tropical regions are expected to differ considerably
from those formed in temperate regions. Although
much information is available about pumice soils
(Egawa, 1977; Maeda et al., 1978; Leamy et al.,
1980; Lowe, 1986; Wada, 1989; Kawasaki et al.,
1991; Dahlgren et al., 1993; Jongmans et al., 1994),
unfortunately it is mostly limited to soils formed in the
middle latitudes or temperate regions.
In Mexico, volcanic ash soils and pumice-derived
soils did not receive attention until the middle of this
century. This recognition stemmed from an urgency to
determine the potential agricultural productivity of
these soils because of the increased demand for food
production. However, in tropical regions of Mexico,
the properties, productivity and classi®cation of soils
formed in pumice or material derived from pumice
that has been reworked and mixed with material from
other sources (volcanogenic pumice alluvium (VPA))
are still a matter for investigation and discussion
(Gama-Castro, 1996). These soils have a great potential for agricultural production as illustrated by the fact
that many of the most productive tropical regions of
Mexico are located on river ¯oodplains close to
volcanic ®elds.
In tropical regions, pumice and other pyroclastic
materials weather rapidly, frequently resulting in the
formation of noncrystalline minerals. Noncrystalline
materials common in VPA include allophane, allophane-like materials, opaline silica and ferrihydrite.
These noncrystalline materials typically occur as coatings on the skeleton (Dahlgren et al., 1993). Because
of the large speci®c surface and high proportion of
reactive sites, a small amount of these noncrystalline
materials can make a disproportionately large contribution to the physical and chemical properties of
these soils. In fact, properties such as water retention
and cation exchange capacity are closely related to
molecular weathering ratio (SiO2/Al2O3) which is a
weathering index in volcanic ash soils (SolleiroRebolledo, 1998).
Weathered pumice-derived soils have morphological, physical, chemical and mineralogical properties
that can be attributed to the in¯uence of a combination
of factors involved in their development including: (1)
very low resistance of the ®nely comminuted pumice
to chemical weathering (Shoji et al., 1993b); (2) high
water retention and cation exchange capacities (Ping
et al., 1989; Par®tt, 1990; Henmi, 1991); (3) low bulk
density (Geist and Strickler, 1978; Bielders et al.,
1990); (4) particle-size distribution and great particle
surface area (Wada, 1986; USDA-SSS, 1994); and (5)
illuviation and neoformation of the short-range order
material, 1:1 phyllosilicates, noncrystalline aluminum
and iron oxides and hydroxides by weathering of
pumice in humid climates (Quantin et al., 1991;
Dahlgren et al., 1993).
In agreement with Osher and Buol (1998), we
consider that ef®cient and sustained use of the pumice
soils requires an understanding of how soil characteristics vary with parent material and their location. We
also assume that to maximize soil productivity and
minimize deterioration, proper management must be
practiced, based on an understanding of the unique
morphological, physical, chemical and mineralogical
properties.
The goals of the present study were: (i) to characterize the soils derived from VPA and estimate the
amount of pumice in the pro®le of selected alluvial
soils formed in the Santiago River ¯oodplain by
optical and physical means; (ii) to establish the in¯uence of weathered pumice in macro and micromorphological, physical, chemical and mineralogical
features of selected alluvial soils; and (iii) to interpret
their genesis, to classify them using Soil Taxonomy
(USDA-SSS, 1994) and to suggest revisions of Soil
Taxonomy where necessary.
2. Site conditions
The study area is situated on the ¯oodplain of the
Santiago River in the coastal plain of west Nayarit,
Mexico (Fig. 1). This area received VPA throughout
the late Pleistocene to recent period (until July 1992).
The various pumiceous deposits can be identi®ed on
the basis of sur®cial geology, stratigraphic position,
geomorphic surfaces, spatial relations through elevation and distance control and soil characteristics such
J.E. Gama-Castro et al. / Soil & Tillage Research 55 (2000) 143±165
145
Fig. 1. Location of soils derived from VPA in west Nayarit, Mexico: P33, Aguamilpa pro®le; P50, Ixcuintla pro®le.
as texture and mineralogy. The Quaternary volcano
San Juan, located on the southwest side of Tepic, State
of Nayarit, is considered the main source of the parent
pumice. Volcanic materials consist of ®ne-grained
dacitic and andesitic pumice deposits (Luhr, 1978).
Both dacitic and andesitic deposits were reworked and
intensively mixed with detrital material, poor in
weatherable minerals and nutrients (Gama-Castro
and Palacios-Mayorga, 1994) with a thickness ranging
from 70 to 150 cm. The 14C data suggest that the
oldest pumiceous deposit is from about 14 700 years
BP (Luhr, 1978).
The area presently is being weathered under a
humid, warm climate with mean January air temperatures between 22 and 238C, and mean July temperatures between 27 and 298C (Table 1). Average
annual precipitation exceeds potential evapotranspiration by about 515 mm per year (Table 1). According
to Par®tt (1990) this annual precipitation (>1200 mm)
leads to the preferential formation of allophane and
allophane-like materials when soluble Si concentrations are low. In the absence of run-off or run-on,
about 400 mm of water percolates through the soil.
The depth of weathering in the soils located in
the study area varies in response to erosion and
sedimentation rates. However, micromorphological
observations as well as mineralogical and chemical
analyses show some evidence of selective weathering
of the primary minerals within 70 cm of the soil
surface. Weathering intensity decreases with soil
depth. In many soils, the groundwater seasonally
¯uctuates, often intersecting the bottom of the C
horizon in the summer. Sometimes, reduced conditions are evident in the deeper layers, with a rH value
19 (rH is related to the redox potential Eh and
corrected for the pH according to FAO-ISRIC-ISSS,
1994).
The soils occur on ¯oodplain landforms and
some river terraces formed by the deposition of
VPA sediments. In these areas the elevations range
from 1.5 to 10 m and the slopes range from 0.5 to 3%
(Table 1).
Deforestation occurred between 50 and 70 years
ago, and the current vegetation consists of grass and
shrubs with a limited number of trees. The original
vegetation was deciduous seasonal forest (Beard,
1955).
The studied soils are among the most important
agricultural soils in the Santiago coastal plain of west
Nayarit. The production of vegetables, tobacco, beans,
rice, sugar cane, and improved pastures support the
dairy industry on these soils.
146
Soila
Horizon development
Location
Slope
(%)
Elevation
(m)
Physiography
Annual mean
temperature
(8C)
Annual mean
rainfall (mm)
Land
use
Parent material
Aguamilpa
Ap±C1±C2, a-tchb, 2Cd±3C
218420 N, 1058170 W
0.5±1.5
10
25
1267.4
Ac
Ixcuintla
Ap±C1, a-tch, 2C±3C
218460 N, 1058140 W
0.5±1.0
2
Flood plains
nearly level
Flood plains
level
26
1227.3
Ac
Volcanogenic alluvium
over detrital deposits
Volcanogenic alluvium
over detrital deposits
a
According to Gama-Castro and Palacios-Mayorga (1994).
Abrupt textural change.
c
Agricultural, tobacco, beans.
b
J.E. Gama-Castro et al. / Soil & Tillage Research 55 (2000) 143±165
Table 1
Pedon locations and site properties
J.E. Gama-Castro et al. / Soil & Tillage Research 55 (2000) 143±165
3. Methods
3.1. Soil selection and ®eld procedures
A semi-detailed survey of soils in west Nayarit,
including Yago and San Blas (Fig. 1), was conducted
over a 10-month period (Gama-Castro and PalaciosMayorga, 1994). This survey was based on mapping
soil units by air photograph interpretation combined
with a ®eld survey. The purpose of this survey was to
supply information, which assisted in taking decisions
about land use and land development planning.
During this survey, 61 soil pro®les were described
from the soil surface to the upper boundary of a speci®ed horizon or root-limiting layer. The vertical section
so de®ned is called soil control section (SCS) according to ICOMAND (1987) and USDA-SSS (1994). The
soil pro®les were described following the guidelines
of FAO-UNESCO (1994) and classi®ed according to
USDA-SSS (1994). These pro®les were sampled in
the ®eld and analyzed in the laboratory. These analyses revealed that 13 of the soils derived from VPA
had unique and complex attributes in the context of
soil production and soil management. Such agronomic
characteristics are related to absorption and adsorption
complexes, aeration, available water capacity, bulk
density, cation and anion exchange capacity, particle-size distribution as well as the presence of amorphous mineral clays in their SCS. These soils showed
high cation exchange and water retention capacities
and low contents of available elements, organic matter
and clay (Gama-Castro and Palacios-Mayorga, 1994).
Based on these previous studies, two typical pedons
representative of such soils were selected for analysis
and soil classi®cation. The pro®les characterize the
two main textural families of soils formed from
different parent materials (USDA-SSS, 1994). These
pedons are numbers P33 and P50. The two pedons are
named after localities in the study area as follows:
P33, Aguamilpa soil, and P50, Ixcuintla soil (Fig. 1).
These soils are intensively used as arable land and
represent some of the most important agricultural soils
found in the Nayarit tropical zone.
3.2. Physical analysis
All physical analyses, except water retention at 33
and 1500 kPa as well as bulk density, were made on
147
air-dried soil samples passed through a 2 mm sieve.
The methods employed are brie¯y described below.
Particle-size distribution analysis: The record of the
volume percentage of material >2 mm was estimated
in the soil descriptions. These fractions were collected
and weighed to determine the percentage of rock
fragments. The sand fractions >47 mm were collected
by sieving on a Fristch Analysette shaking apparatus
before removing the organic matter with 30% H2O2.
Fractions 0.6 or 2.0 mm) in the whole soil in studied pedons: (1) reworked pyroclastics; (2) glass
aggregates; (3) detrital sediments. US: upper section; LS: lower section.
separate strata of Holocene alluvium. Such discontinuities show the presence of intermittent alluvial
sediment deposition by Santiago River ¯ooding,
spread some hundreds or thousands of year apart.
Stratum 1, named as the upper section, contains the
earliest pyroclastic deposits of VPA and includes Ap
and pedogenetically altered C horizons. Stratums 2
and 3, the lower section, are unaltered fragmental
deposits that include the 2C and 3C layers, respectively (Fig. 2; Table 2).
In the upper section of both pedons, rock fragments
(5.0±2.0 mm in diameter) represent a signi®cant fraction of the whole soil (26±39% by weight, Table 3).
They are mainly constituted by reworked pyroclastic
sediments mixed with ordinary detrital sediments and
a certain amount of organically derived material. As
shown in Fig. 2, the detrital clastics (lithic fragments)
such as cobbles and pebbles are relatively scarce in
this section (15±17% by volume of the whole soil).
Many of these lithic fragments have lost their original
internal fabric and subrounded shape and exhibit some
compressed structures and irregular shapes, suggesting fragmentation and deformation. A great percentage of reworked pyroclastics and lithic fragments
have a red- to yellow-colored chemical alteration rind,
indicating that weathering has affected these materials.
In contrast, the layers of the lower section contain
many round and subrounded pebbles, cobbles and
some stones that constitute more than 45% by volume
of the whole soil. The fraction ®ner than 2.0 mm is
sandy as de®ned for the sandy particle-size class
(Table 3). Such layers satisfy all the requirements
for a sandy skeletal class. In addition the number of
lithics with an alteration rind is very low (